The present invention relates generally to gas turbine engines, and, more specifically, to variable area exhaust nozzles therefor.
One type of turbofan gas turbine engine for powering an aircraft in flight includes an afterburner or augmenter for providing additional thrust when desired, with a variable area exhaust nozzle disposed at the aft end thereof. Since the engine operates at varying power levels including idle, cruise, and maximum afterburner, the exhaust nozzle is suitably adjustable for optimizing performance with maximum efficiency.
A typical variable area exhaust nozzle is an assembly including a converging duct or nozzle defined by a plurality of circumferentially adjoining primary exhaust flaps pivoted at their leading edges to an outer casing. A diverging duct or nozzle is defined by a plurality of circumferentially adjoining secondary exhaust flaps pivoted at their leading edges to the trailing edges of the primary flaps. The trailing edges of the secondary flaps are pivotally joined to a plurality of circumferentially adjoining outer flaps which in turn are joined to the outer casing. The converging-diverging (CD) nozzle includes an inlet at the entrance to the converging nozzle, a throat of minimum flow area, designated A.sub.8, at the juncture between the converging and diverging nozzles, and an outlet having a larger flow area, designated A.sub.9.
During operation, suitable actuators pivot radially inwardly and outwardly the primary flaps to adjust the angle of convergence and the throat area, and in turn adjust the angle of divergence of the secondary flaps and the outlet area. In this way, the exhaust gases from the engine may be accelerated in the converging nozzle to a choked velocity of Mach 1 at the throat, and then expanded in the diverging duct at supersonic velocities for enhanced performance.
The resulting variable area exhaust nozzle is relatively complex in construction and requires many individual components pivotally joined together, and adjusted in position using suitable actuators and linkages. The individual primary and secondary flaps must be suitably cooled and sealed at their junctions to control undesirable leakage of the exhaust gases therebetween.
Fixed area exhaust nozzles are also known but are used in less demanding applications. For example, a simple converging nozzle may be used without a cooperating diverging nozzle, with fixed inlet and throat flow areas. Or, a fixed diverging nozzle may be used in conjunction with the fixed converging nozzle with the flow areas at the inlet, throat, and outlet also being fixed and therefore optimized for only a single region of engine performance. Fixed area CD nozzles are therefore not practical or desirable for an aircraft engine operating over a wide range of power in its flight envelope.
Accordingly, it is desired to have a relatively simple fixed CD exhaust nozzle with variable flow area capability for enhancing engine performance without complex area control mechanisms and attendant weight.